I have the same motherboard (Z370 Aorus Ultra Gaming 2.0) and can confirm that there is no "Other OS" setting available. BIOS version of my board is F11.
Have you tried to install High Sierra or Mojave with the first "Windows 8/10" option selected?
I have the same motherboard GA Z370-HD Bios version F10 and with the option Windows 8/10, not Windows 8/10 WHQL, High Sierra, Mojave and Catalina beta 1 to 6 work fine
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